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Thermally induced conformational transition of double‐stranded xanthan in aqueous salt solutions
Author(s) -
Kitamura Shinichi,
Takeo Kenichi,
Kuge Takashi,
Stokke Bjørn T.
Publication year - 1991
Publication title -
biopolymers
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.556
H-Index - 125
eISSN - 1097-0282
pISSN - 0006-3525
DOI - 10.1002/bip.360311102
Subject(s) - chemistry , enthalpy , aqueous solution , endothermic process , differential scanning calorimetry , polyelectrolyte , analytical chemistry (journal) , glass transition , calorimetry , crystallography , thermodynamics , polymer , chromatography , organic chemistry , physics , adsorption
The thermally induced conformational transition of double‐stranded xanthans (degree of pyruvate substitution, DS P = 0.45) having M w = 3.1, 5.7, and 20.3 × 10 5 has been studied in aqueous salt solutions by high‐sensitivity differential scanning calorimetry (DSC). The double strandedness of these samples in the ordered conformation was ascertained by the value of mass per unit length, M L = 2090 ± 270 g mol −1 nm −1 , which was determined from the contour length obtained by electron microscopic observations and the molecular weight by light scattering measurements. The temperature at half completion of the transition T ½ for these samples increased linearly with the logarithm of the cation (Na + , K + ) concentration. The plot of 1/ T ½ vs the natural logarithm of cation (Na + ) concentration in m M for the sample with M w = 5.7 × 10 5 (15‐SX) yielded the equation 10 3 / T ½ = 3.45−0.159 in[Na + ]. The specific enthalpy Δ h cal , for 15‐SX, essentially independent of salt concentration above 20 m M , was 8.31 ± 0.39 J/g (SD, n = 6). No systematic dependence of molecular weight on the transition temperature and the enthalpy was observed. Application of the Manning polyelectrolyte theory to the system using the DSC data suggested that the separation of the double strand of xanthan into two single chains was not completed at the temperature where the endothermic peak was finished. This suggestion is consistent with recent findings by light scattering measurements as a function of temperature. Our DSC study was extended to include four other samples from various sources. It was found that T ½ and Δ h cal depend on the pyruvate contents of the samples. For example, the t ½ ( t ½ /° C = T ½ /K − 237.15) values for samples with high pyruvate content (DS p = 0.9) and depyruvated (DS p = 0.14) in 20 m M aqueous NaCl were 48.8 and 85.3°C, respectively. Two other samples showed relatively broad DSC curves having shoulders, which were resolved into two independent components. Thermodynamic parameters for each component were examined as a function of salt concentration, and the results obtained were interpreted in terms of the heterogeneity of the pyruvate content of the samples.